In recent years, a package into which an optical source such as a laser diode and an optical modulator for modulating light from the optical source are integrated may be mounted on an optical module that sends and receives optical signals. The package is arranged on a board such as a Printed Circuit Board (PCB), and converts electrical signals, which are input from a driver connected via electrodes on the board, into optical signals.
The connection between the package and the electrodes on the board is realized by soldering, for example, leads projecting from a bottom face of the package to the electrodes on the board. The package and the board may be connected via a flexible board (FPC: Flexible Printed Circuit) with appropriate electrode pattern design in order to restrain impedance mismatches at connecting parts. In other words, for example, electrodes on the FPC board are soldered to leads that project from side surfaces of the package, and further the electrodes on the FPC board and the electrodes on the board are soldered to each other.
Lithium niobate (LiNbO3 (LN)) is used as material of an optical modulator included in the package in many cases, however, indium phosphide (InP), a silicon photonic integrated circuit (Si—PIC), or the like is introduced to miniaturize the optical modulator in these days. By using these materials, the optical modulator is miniaturized, and thus the package can be miniaturized.
In order to miniaturize the package, it is desirable that the size of an electrical interface that connects the package to the board or the FPC board is reduced. On this point, as described above, when the leads that project from the bottom face of the package are used as the electrical interface, restriction on design occurs, for example, a gap is provided between pads for brazing the leads to the package, and therefore, there is limitation in reducing pitches between the leads. Also, when the FPC board is used, restriction on design occurs, for example, lands are arranged around through-holes into which the leads that project from side surfaces of the package are inserted, and therefore, there is limitation in reducing pitches between the leads.
Therefore, a configuration is studied in which, instead of using the leads, a ceramic board is exposed to the outside of the package and terminals on the ceramic board are used as the electrical interface. In such a configuration, the terminals on the ceramic board can be directly soldered to, for example, the electrodes on the FPC board, and thus pitches between the terminals can be adjusted to decrease comparatively easily. As a result, it is possible to miniaturize the package and to provide the miniaturized optical module.
Patent document 1: Japanese Laid-open Patent Publication No. 2001-230506
Patent document 2: Japanese Laid-open Patent Publication No. 2012-47823
Patent document 3: Japanese Laid-open Patent Publication No. 2014-89400
Patent document 4: Japanese Laid-open Patent Publication No. 2013-153136
However, there is a problem that work efficiency in manufacturing is not sufficient when the terminals on the ceramic board and the electrodes on the FPC board are soldered to each other. Specifically, because a part that is exposed from the package of the ceramic board is small, it is difficult to fix the FPC board to the part and solder the terminals and the electrodes to each other. To solve this, to enlarge the exposed part by making the ceramic board project largely from the package may be considered, however in this case, the terminals on the ceramic board are elongated. As a result, high-frequency propagation loss increases, thereby leading to degradation of frequency bands of the transmitted electrical signals. In addition, the elongation of the terminals leads to enlargement of area of soldered regions, and thus the possibility of occurrence of impedance mismatches increases. Therefore, it is desirable that the efficiency of soldering to the FPC board is improved without enlarging the exposed part of the ceramic board.
Moreover, in such a case that the package is miniaturized, the pitches between the terminals on the ceramic board decrease, however, the decrease in the pitches between the terminals leads to difficulty in positional adjustment between the terminals and the electrodes on the FPC board. Specifically, in such a case that the terminals on the ceramic board and the electrodes on the FPC board are directly soldered, there is no means for fixing the position of the FPC board, for example, a means for inserting the leads projecting from the package into through-holes of the FPC board. For this reason, also on this point, soldering of the terminals on the ceramic board and the electrodes on the FPC board is accompanied by a decrease in the work efficiency.